Chapter 8 Astronomical Control

Astronomical Control of Solar Radiation

Lesson Goals

• Understand key aspects of the Earth’s movements:

  • Rotation

  • Orbit

  • Equinox

  • Solstice

• Name and explain the three orbital cycles:

  • Obliquity

  • Precession

  • Eccentricity

• Understand how the orbital cycles impact climate:

  • Impacts on different latitudes

From Tectonics to Orbital Scale Change

• Movement from tectonic-scale change (tens to hundreds of millions of years) to orbital scale change (last few million years)

• Focus on the last ~3 million years:

  • Rich proxy records available that provide insights into climate response to these changes

  • Includes records of forcing and responses

Orbital Scale Changes (aka Milankovitch Cycles)

• Occur over 20,000 to 400,000 years:

  • Obliquity: Refers to the axial tilt of the Earth

  • Eccentricity: Describes the shape of the Earth’s path around the Sun

  • Precession: Indicates the Earth’s wobble

  • Change in the direction that the axis points

  • Change in the major and minor axes of the ellipse over time

Earth’s Tilt Today

• The Earth currently rotates at an angle of 23.5º from the plane of the ecliptic

• One rotation on the axis equals 24 hours

Seasons

• Seasons are determined by:

  • Earth’s tilt

  • Changing position around the Sun

• Solstices:

  • June 21:

  • Northern Hemisphere summer

  • Southern Hemisphere winter

  • December 21:

  • Northern Hemisphere winter

  • Southern Hemisphere summer

• Equinoxes: Defined as “equal night”

  • March 20:

  • Northern Hemisphere spring

  • Southern Hemisphere autumn

  • September 22:

  • Northern Hemisphere autumn

  • Southern Hemisphere spring

Changes in Axial Tilt (Obliquity)

• The axial tilt does not remain constant over time

• Cycle Duration:

  • 41,000-year cycle

• Impact of larger tilt on seasons should be understood:

  • Greater axial tilt results in more extreme seasons (hotter summers, colder winters)

Orbit Around the Sun

• Perihelion:

  • Closest point to the Sun

  • Distance: 153 million km

  • Occurs on January 3rd

• Aphelion:

  • Farthest point from the Sun

  • Distance: 158 million km

  • Occurs on July 4th

Changes in Orbit: Eccentricity

• Eccentricity varies from nearly circular to elliptical

• Increase in eccentricity means greater differences in the axes

• Current trend towards a more circular orbit

• Cycle Duration:

  • ~100,000-year cycle

Precession of the Solstices and Equinoxes

• Definition of Precession:

  • Described as a long-term wobbling motion of the axis

  • Changes both the positions of the solstices and equinoxes

• Analogy:

  • Similar to a spinning top around a tilted axis, where the Earth wobbles and gradually leans in various directions over time

Drivers of Precession

• Precession is a result of:

  • Precession of the axis:

  • Direction the axis points rotates approximately every 25,700 years

  • Precession of the ellipse:

  • The entire elliptical orbit rotates

  • Combined effect leads to:

  • Precession of the equinoxes: cycles approximately every 23,000 years

Timeline of Precession Effects

• Dates and distances pertinent to precession include:

  • June 21:

  • Position today, returning to a moderate position after 23,000 years

  • December 21 (solstice):

  • Maximum distance from the Sun (aphelion)

  • Minimum distance from the Sun (perihelion)

Precessional Index and Solar Radiation

• Definition of Precessional Index:

  • Combined effects of eccentricity and precession on solar radiation received

• Effects of eccentricity on Earth-Sun distance occur during the 23,000-year precessional cycle

• Seasonal solar radiation changes are influenced by Earth-Sun distances

• High Eccentricity:

  • Largest contrasts in Earth-Sun distances

• Low Eccentricity:

  • Smallest contrasts in Earth-Sun distances

Insolation Curves

• June and December insolation curves indicate predominance of the 23 ka cycle at lower and mid-latitudes

• Notable absence in high latitudes during winter due to factors such as:

  • Lack of evidence for 41 ka cycle (obliquity/tilt) at lower latitudes

Searching for Orbital Scale Changes in Climate Records

• Methods for Analysis:

  • Time series analysis

  • Spectral analysis

  • Filtering of cycle amplitudes

• Key cyclical durations analyzed:

  • 100,000 years

  • 41,000 years

  • 23,000 years

• Spectral Analysis Findings:

  • Sine-wave cycles possess line spectra showing distinct amplitudes

  • Actual climate records display peaks across a broader range of periods

Recap of Lesson Goals

• Reinforce understanding of Earth movements:

  • Rotation

  • Orbit

  • Equinox

  • Solstice

• Review of the three orbital cycles:

  • Obliquity

  • Precession

  • Eccentricity

• Understanding of orbital cycles’ impacts on climate across different latitudes